An image displayed on a screen of an information processor, such as an image on each page of an electronic book, can be copied by taking a screen shot. That is, image data of each page of the electronic book can be obtained by storing display data stored in a display memory.
In an attempt to prevent such fraudulent acts, Patent Literature 1 below discloses a technology for randomly selecting S for each pixel of an original image, generating a first converted image obtained by adding S to each pixel value and a second converted image obtained by subtracting S from each pixel value, and displaying these two converted images while switching them at a high speed. In accordance with this technology, even though a viewer recognizes as if the original image is displayed, a first or a second converted image is obtained by a screen shot, and thus it is possible to prevent fraudulent acquisition of original image data.